One of the important aspects to note about this vital scene in the play is the way that tension is greatly raised by the disrupted nature of Lady Macbeth's words. As she talks to herself, various sounds startle her causing her speech to become interrupted with inerjections such as "Hark!" The first time this happens, it is just the owl that she hears, but note how she interprets the sound:
It was the owl that shriek'd, the fatal bellman,
Which gives the stern'st good-night. He is about it.
The reference to the "stern'st good night" is of course a reference to the death that Macbeth is busy committing as she listens. When her husband returns, she comments that all she heard was "the owl scream, and the crickets cry." The normal nighttime sounds of nature is all that she hears, but at the same time what she and her husband are doing imbue those sounds with a far more sinister significance.
The sounds that Lady Macbeth hears in Act 2 scene 2 are heavily symbollic, as they suggest her role as both a witch and an 'other'. This is seen through the use of familiars such as 'Paddock' and 'Graymalkin' - the 'owl' that 'shreiked' uses anthropmorphism in order to suggest the pain of the natural world at Duncans death. She is suggested to be a witch, as the owl was both a harbinger of death, and a creature which had strong associations to a Shaksepearan audience of witchcraft. Equally the noises are seen to be significant as they forebode her descent into maddness, the fact that the 'crickets cry' indicative through alliteration of their pain and disparity. This mirrors her shaken conciense about the crime she has just commited. Her language and reaction to noise suggests her fragile emotions and the empty words which she has previously greeted Macbeth with. This is perhaps a suggestion by Shakspeare that women are not built for the crime of murder and its consquence. Lady Macbeth has not truely been 'unsexed.'